The invention relates to screen printing machines in general, and more particularly to improvements in screen printing machines with movable screens.
It is already known to provide the single printing unit or each printing unit of a screen printing machine with a movable screen, one or more squeegees, a back support (e.g., a flat bed or a cylinder), a mechanism for transporting a web of paper, textile or other material along a predetermined path between the screen and the back support of each printing unit, and means for monitoring the positions of passer marks if the machine is a multicolor printing machine. Certain multicolor screen printing machines are constructed in such a way that the web advances from printing unit to printing unit to receive printed matter in two or more different colors. Alternatively, the web is rolled up after having been advanced through a first printing unit and is unrolled at a later time for advancement through another printing unit. The first outlined mode of operation is preferred at this time. A web which is provided with printed matter in two or more printing units is provided with passer marks (also called alignment marks or registration marks) which are monitored, and the thus obtained signals are utilized to effect appropriate adjustments of the web and to thus ensure the application of printed matter to predetermined portions of the web. Reference may be had, for example, to commonly owned U.S. Pat. No. 4,964,335 granted Oct. 23, 1990 for "Screen printing method" which describes and shows a machine with means for adjusting the position of a web in dependency on signals which are generated as a result of monitoring passer marks on the web. Applicators of passer marks are disclosed in commonly owned U.S. Pat. No. 4,510,864 granted Apr. 16, 1985 for "Screen printing machine" and in commonly owned U.S. Pat. No. 4,249,688 granted Feb. 10, 1991, "Device for intermittent feeding of webs" to which reference may be had, if necessary. Accurate positioning of the web in a multicolor printing machine is desirable and advantageous in order to ensure that each next-following application of printed matter takes place in exact accordance with a preselected pattern. This reduces the likelihood of undesirable shifting of margins which would entail the formation of improperly colored marginal prints.
Heretofore known procedures which involve reliance on passer marks for accurate positioning of the web relative to the screens of a multicolor printing machine necessitate a multi-stage displacement of the web including rapid advancement for coarse adjustment and much slower additional adjustment for precise positioning of the web prior to application of ink in the second, third, etc. printing unit of the machine. Thus, the machine must be provided with means for controlling the operation of the web advancing mechanism with a very high degree of accuracy and in a plurality of stages. If the multicolor screen printing machine is of the type wherein the web is caused to form loops between successive printing stations, the transporting mechanism is or can be designed to increase or reduce the sizes of the loops between successive printing units. If the just described multicolor printing machine employs counterpressure rollers or cylinders, rotation of the rollers at a higher or lower speed and/or through a larger or smaller angle will entail a change in the dimensions of the loop which is formed by the web of material to be provided with printed matter. A drawback of such proposals is that the tensional stresses upon the web as well as the slippage of the web relative to a counterpressure cylinder constitute two variables which are sufficiently unpredictable to appreciably affect the quality of the ultimate product. Therefore, the just outlined screen printing machines are not suitable for the application of printed matter with a very high degree of accuracy (e.g., in the micro range) such as is desirable, for example, in connection with the application of printed matter to circuit boards and the like.
German patent application No. 30 15 159 A1 of Ericsson (published Nov. 6, 1980) discloses a screen printing machine which is provided with optical sensors for detection of passer marks or color shifts. It has been found that such machines are incapable of ensuring highly accurate adjustments of the positions of screens and of the material to be printed relative to each other. On the other hand, the demand for highly accurate screen printing machines is on the increase, not only in connection with the application of printed matter for the purpose of making printed circuit boards but also for a number of other applications.
European patent application No. 0 259 776 of Amao et al. (published Mar. 16, 1988) discloses a screen printing machine wherein the position of the material to be printed can be adjusted in three different directions in response to signals which are generated in response to monitoring of passer marks or alignment marks. The monitoring means includes a camera which is installed for movement in a direction at right angles to the direction of advancement of the material to be printed.
German patent application No. 37 07 866 A1 of Roch et al. (published Oct. 1, 1987) discloses a method of and an apparatus for controlling and adjusting the components of a printing and cardboard making machine. The inventors propose to gather information from the components to be adjusted, to evaluate the thus obtained information, to store the information, to utilize the stored information for presentation in alphanumerical and graphical form, to display the information on a screen, and to manually contact the displayed information for the purpose of effecting adjustments of various components of the machine.
European patent application No. 0 311 729 A1 of Zimmer et al. (published Apr. 19, 1989) discloses a method of positioning cylindrical printing components in a machine with at least two printing stations. The positioning of cylindrical components is effected in response to monitoring of passer marks.